1. Field of the Invention
The present invention relates to a thread-forming screw, in particular, to a thread-forming screw produced of a corrosion-resistant metal and having a stem, at least one thread arranged, at least regionwise, over a circumference of the stem, and a plurality of cutting elements arranged in a respective plurality of recesses formed in the at least one thread, the cutting elements having a hardness greater than a hardness of the at least one thread.
2. Description of the Prior Art
Thread-forming screws of the type described above have a stem at one end of which, there is provided a head that serves as load application means. In thread-forming screws or thread-cutting screws, the outer thread has a high strength which permits to form a female thread in a constructional component. To this end, at least the steel of the outer thread is hardened. For external applications, the screws, such as concrete screws, are formed of corrosion-resistant steels. Such materials usually cannot have their strength increased by heat treatment to a sufficient degree that would insure a reliable formation of the female or inner thread in concrete.
German Publication DE 198 15 670 A1 discloses a thread-forming screw which is formed of a corrosion-resistant steel that is partially dispersion hardened.
The drawback of this screw consists in that the maximum achievable hardness is not sufficient for insertion of these screws in a hard mineral material, e.g., concrete.
International Publication WO 94/25764 discloses a thread-forming screw formed of a corrosion-resistant steel which is provided with a coating over its entire surface and which is formed by an ion-nitration process. The drawback of this screw consists in that the thickness of the coating is maximum 0.2 mm. When such screws are used in hard mineral materials, e.g., for insertion in constructional components formed of such materials, the coating is rubbed off already after several revolutions of the screw.
German Publication DE 198 52 338 A1 discloses a screw formed of a stainless steel and the thread of which is provided with bores for receiving pin-shaped cutting inserts formed of a hardened steel and which provide for thread-cutting in constructional components formed of hard mineral materials. The drawback of the above-described screw consists in that the manufacturing of these screws is very expensive because a plurality of pin-shaped cutting inserts should be driven into the receiving bores. At both ends of the pin-shaped cutting insert, there is provided, respectively, a cutting head to be able to drive the cutting inserts into the receiving bores in both directions. After the pin-shaped cutting inserts are driven into the receiving bores, the cutting heads, which project above the bores, should be ground. In the radial direction, the pin-shaped cutting inserts are held in the receiving bores only by friction.
International Publication WO 2004/074697 A1 discloses a thread-forming screw having a stem formed of a material with a carbon content between 0% and 0.5% by weight and with a thread formed integrally with the stem. In the thread, there is embodied a cutting element that is formed as a weld body of a metal with a carbon content of more than 0.8% by weight and which is fixedly connected with the stem. The weld body is formed of a high-speed steel and is welded into the thread. The drawback of this screw consists in that it is too costly to weld into a thread separate weld bodies. In addition, a steel with a carbon content of above 0.8% by weight, so-called hyper-eutectoid steel, has a poor weldability, so that with securing the cutting element in the thread, a not unsubstantial region of the thread and of the core are weakened during the welding process, which can result in formation of fissures in this region.
Accordingly, an object of the present invention is a thread-forming screw in which the drawbacks of the known thread-forming screws are eliminated.
Another object of the present invention is a thread-forming screw capable of cutting a thread in constructional components formed of a hard material, e.g., such as concrete.